Modern refineries employ many upgrading units such as fluidic catalytic cracking (FCC), hydrocracking (HCR), alkylation, and paraffin isomerization. As a result, these refineries produce a significant amount of isopentane. Historically, isopentane was a desirable blending component for gasoline having a high octane (92 RON), although it exhibited high volatility (20.4 Reid vapor pressure (RVP)). As environmental laws began to place more stringent restrictions on gasoline volatility, the use of isopentane in gasoline was limited because of its high volatility. As a consequence, the problem of finding uses for by-product isopentane became serious, especially during the hot summer season. Moreover, as more gasoline compositions contain ethanol instead of MTBE as their oxygenate component, more isopentane must be kept out of the gasoline pool in order to meet the gasoline volatility specification. So, the gasoline volatility issue becomes even more serious, further limiting the usefulness of isopentane as a gasoline blending component.
The process of the present invention solves this problem by converting undesirable isopentane to low-RVP gasoline blending components by alkylation of the isopentane with a refinery stream containing ethylene using an ionic liquid catalyst. Other olefins, such as propylene, butylenes, and pentenes can also be used to convert isopentane to make low RVP hydrocarbon product. By reducing the excess isopentane, the burden of storing isopentane and/or concerns for isopentane usage are eliminated.
In general, conversion of light paraffins and light olefins to more valuable cuts is very lucrative to the refining industries. This has been accomplished by alkylation of paraffins with olefins, and by polymerization of olefins. One of the most widely used processes in this field is the alkylation of isobutane with C3-C5 olefins to make gasoline cuts with high octane number using sulfuric and hydrofluoric acids. This process has been used by refining industries since the 1940's. The process was driven by the increasing demand for high quality and clean burning high octane gasoline.
Commercial paraffin alkylation processes in modern refineries use either sulfuric acid or hydrofluoric acid as catalyst. Both of these processes require extremely large amounts of acid to fill the reactor initially. The sulfuric acid plant also requires a huge amount of daily withdrawal of spent acid for off-site regeneration. Then the spent sulfuric acid is incinerated to recover SO2/SO3 and fresh acid is prepared. The necessity of having to handle a large volume of used acid is considered a disadvantage of the sulfuric acid based processes. On the other hand, an HF alkylation plant has on-site regeneration capability and daily make-up of HF is orders of magnitude less. However, the aerosol formation tendency of HF presents a potentially significant environmental risk and makes the HF alkylation process less safe than the H2SO4 alkylation process. Modern HF processes often require additional safety measures such as water spray and catalyst additive for aerosol reduction to minimize the potential hazards.
Although these catalysts have been successfully used to economically produce the best quality alkylates, the need for safer and environmentally-more friendly catalyst systems has become an issue to the industries involved. The ionic liquid catalyst of the present invention fulfills that need.
In addition, implementing the present invention relieves a refinery of the problem and waste associated with excess fuel gas production. It does this by using ethylene in, for example, offgas from a FCC unit as the source of olefins for the alkylation of isopentane. Typically FCC offgas contains ethylene up to 20 vol %. Other olefin streams containing ethylene or other olefins such as coker gas could also be used for this process. The overall gasoline volume is increased by this process of invention. The net amount of fuel gas from the FCC de-ethanizer is reduced, thus lowering the burden of fuel gas processing equipment. A further benefit of the present invention is that extracting ethylene will improve the purity of hydrogen in FCC offgas. The improved concentration of hydrogen in the offgas may allow the economical recovery of pure hydrogen with the use of a hydrogen recovery unit, such as a pressure-swing adsorption (PSA) unit or a selective hydrogen-permeable membrane unit. Considering tight environmental regulations associated with fuel gas production and shortage of hydrogen in modern refineries, the benefits of fuel gas reduction and hydrogen production are highly desirable.
The most economical, thus most desirable, olefin streams are FCC de-ethanizer overhead containing hydrogen, methane, ethane, and ethylene, or coker gas containing olefins. The present process converts the isopentane stream to a low RVP dimethyl pentane and trimethyl butane gasoline fraction with little octane loss. By employing the process of the invention, the overall gasoline volume produced at a refinery is increased. In addition, the net amount of fuel gas from the FCC de-ethanizer is reduced, thus lowering the burden on the fuel gas processing equipment.
Furthermore, the present invention includes a new paraffin alkylation process which can produce alkylate gasoline, the most desirable blending component in gasoline, in an environmentally sound manner far superior to the conventional alkylation process. In comparison with the conventional processes, the process according to the present invention offers the following significant advantages over conventional alkylation:                Substantial reduction in capital expenditure as compared to sulfuric acid and hydrofluoric acid alkylation plants        Substantial reduction in operating expenditure as compared to sulfuric acid alkylation plants        Substantial reduction in catalyst inventory volume (potentially by 90%)        A substantially reduced catalyst make-up rate (potentially by 98% compared to sulfuric acid plants)        A higher gasoline yield        Comparable or better product quality (Octane number, RVP, T50)        Significant environment, health and safety advantages        Expansion of alkylation feeds to include isopentane and ethylene.        
It follows that employing a process according to the present invention a refiner can upgrade both isopentane and ethylene and at the same time react conventional alkylation feed components (e.g., butene, propylene, pentene, and isobutane) to produce high quality gasoline blending components. These additional capabilities are made possible in part with the high activity and selectivity of the ionic liquid catalyst used for these reactions. The present invention provides its greatest benefits when all these alkylation reactions are conducted with ionic liquid catalysts and none are conducted using sulfuric acid or hydrofluoric acid catalysts.